The Python interpreters are patched so they can build .pyc bytecode free
of certain indeterminism.
When building Python packages we currently set
```
compiling python files.
in nix store.
DETERMINISTIC_BUILD=1;
PYTHONHASHSEED = 0;
```
Instead if setting these environment variables in the function that
builds the package, this commit sets the variables instead in the Python
setup hook. That way, whenever Python is included in a derivation, these
variables are set.
See also the issue https://github.com/NixOS/nixpkgs/issues/25707.
A package set is constructed for a specific interpreter. Therefore, we add the
possibility to override the package set to the interpreter. This should make it
easier to override the interpreter and the package set at the same time.
